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The displacement of synchronous generators with inverter-based sources in the electric grid can results in larger frequency deviations due to lower rotating inertial energy. Existing grid-tied inverters operate as grid-following sources that track the voltage angle of the grid to control their output. Even with inverter fast frequency support, frequency regulation still depends on the remaining synchronous generators. In contrast, grid-forming sources actively control their frequency output, making it possible for them to naturally support the system frequency while sharing a portion of the load change. In this work, the impact of grid-forming and grid-following inverter controllers on the system frequency dynamics is studied and compared. A simple two-source system is modeled, and the small-signal dynamic characteristics are studied at various penetration levels and varying levels of mechanical inertia. The resulting eigenvalue trajectories are presented and these analytical results are validated by the corresponding time-domain plots. The results exhibit significant dynamic response advantages of the grid-forming inverter over its grid-following counterpart, particularly under the most demanding conditions when the penetration level of inverter-based sources reaches high levels and the available system stored inertial energy drops.
Pattabiraman et al. (Wed,) studied this question.